(1) Field of the Invention
The present invention relates to a method for performing diagnostics on a system. More specifically, the present invention relates to a method for diagnosing performance changes in gas turbine engines.
(2) Description of Related Art
The goal of Gas Turbine Performance Diagnostics is to accurately detect, isolate and assess the changes in engine module performance, engine system malfunctions and instrumentation problems from knowledge of measured parameters taken along the engine's gas path. Discernable shifts in engine speeds, temperatures, pressures, fuel flow, etc., provide the requisite information for determining the underlying shift in engine operation from a presumed reference (nominal) state.
Engine performance changes manifest themselves in one of two ways: a)gradual (long term) deterioration or b)rapid (short-term) deterioration or performance restoration. This observation arises from the nature of engine design. Engines are designed to be robust and last. Hence, engines exhibit a gradual accumulation of component damage. However, fit clearances and externally caused damage can change the mode of damage to excessive wear and crack propagation. Such effects may be initially very small but can lead to part failures.
For example, the wear of a bleed linkage may initially cause bleed sticking and scatter in the data due to improper operation, but the eventual linkage failure leads to a step change in performance. Similarly crack propagation can lead to blade tip failures collateral damage, and rapid ramping or even step changes in module deterioration. On-wing and shop maintenance performance restoration shows up as an abrupt improvement in performance. Likewise, part of a production engine's performance is lost abruptly on its first flight because that is when the engine rub-in occurs and sets its initial running clearances.
These modes of damage accumulation affect component (module) performance and engine sub-systems such as bleeds, cooling flows, variable geometry mechanisms, etc. in ways that can be classified in terms of estimated changes in parameters such as efficiencies, flow capacities, and effective nozzle areas. Gradual damage accumulation occurs in all components gradually and requires multiple fault isolation (MFI). Rapid performance degradation is usually traced to a single module or system and sometimes a second module affected by collateral damage and requires single fault isolation (SFI). Rapid performance improvements due to maintenance actions can be traced to the individual modules and systems repaired and isolated appropriately.
There are a number of algorithms available to address the problem of estimating a) the mode of gradual deterioration as well as b) the mode of rapid deterioration in gas turbine system performance. These methods constitute MFI (Multiple Fault Isolation) and SFI (Single Fault Isolation). The former implies that all of the engine components (whose shifts in performance we are estimating) are deteriorating slowly whereas the latter implies a concession, i.e. that a rapid trend shift is most probably due to a single root cause (or perhaps two when there is an interaction or collateral damage).
Typically each form of degradation is diagnosed separately as it occurs. Often a single fault is analyzed with an MFI system because reliable SFI systems are not available. Partitioning SFI and MFI analyses is necessary because an error occurs when single faults are analyzed by MFI algorithms or conversely, when multiple faults are analyzed by SFI methods. In the former case, the single underlying cause is smeared across a plurality of faults with an attendant attenuation and possible lack of identifiability. In the latter case, the gradual degradation in multiple components is erroneously assigned to a single cause. Current diagnostic systems operate without this necessary partitioning and as a consequence do not provide reliable closure of engine performance in test cells and flight.
What is needed is a method that will bring closure between production performance, flight performance, maintenance actions, and shop performance. Preferably, such a method would separate and independently sum the rapid and gradual performance changes experienced by an engine.